Cathode ray tubes (CRTs) have been widely used for display devices, such as televisions and computer monitors. However, CRTs have the disadvantages of being large, heavy, and requiring a high drive voltage. As a result, flat panel displays (FPDs) that are smaller, lighter, and require less power have grown in popularity. Liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, and light emitting diode (LED) devices are some of the types of FPDs that have been introduced in recent years.
The LED device may either be an inorganic LED device or an organic LED (OLED) device depending upon the source material used to excite carriers in the device. OLED devices have been particularly popular because they have bright displays, low drive voltages, and can produce natural color images incorporating the entire visible light range. Additionally, OLED devices have a preferred contrast ratio because they are self-luminescent. OLED devices can easily display moving images because they have a short response time of only several microseconds. Moreover, such devices are not limited to a restricted viewing angle as other LED devices are. OLED devices are stable at low temperatures. Furthermore, their driving circuits can be cheaply and easily fabricated because the circuits require only a low operating voltage, for example, about 5V to 15V DC (direct current). In addition, the process used to manufacture OLED devices is relatively simple.
FIG. 1 is a circuit diagram of an active matrix OLED (AM-OLED) device according to the related art.
In FIG. 1, one pixel region of an AM-OLED device is composed of a switching TFT T1, a driving TFT T2, a storage capacitor Cst, and an OLED 10. A gate electrode of the switching TFT T1 is connected to a gate line GL, the source electrode of the switching TFT T1 is connected to a data line DL, and the drain electrode of the switching TFT T1 is connected to a gate electrode of the driving TFT T2. The source electrode of the driving TFT T2 is connected to a power line VDD, and the drain electrode of the driving TFT T2 is connected to an anode of the OLED 10. A cathode of the OLED 10 is grounded. The storage capacitor Cst is connected to the gate and source electrodes of the driving TFT T2. When a scanning signal is applied to the gate electrode of the switching TFT T1 through the gate line GL and an image signal is applied to the drain electrode of the switching TFT T1 through the data line DL, the switching TFT T1 is turned ON. The image signal is stored in the storage capacitor Cst through the switching TFT T1. The image signal is also applied to the gate electrode of the driving TFT T2. As a result, a turn-on rate of the driving TFT T2 is determined. The current that passes through the channel of the driving TFT T2 in turn passes through the OLED 10 causing the OLED 10 to emit light in proportion to the current density. Since the current density is proportional to the turn-on rate of the driving TFT T2, the brightness of the light can be controlled by the image signal. The driving TFT T2 may be driven by charge stored in the storage capacitor Cst even when the switching TFT T1 is turned OFF. Accordingly, the current through the OLED 10 is persistent until a next image signal is applied. As a result, light is emitted from the OLED 10 until the next image signal is applied.
In FIG. 1, for example, the switching TFT T1 and the driving TFT T2 correspond to a positive channel metal oxide semiconductor (PMOS) TFT.
Meanwhile, since a driving current is applied to the OLED device through the power line, a pixel current always flows into the power line. Accordingly, the value of the driving current is increased as the number of pixels is increased.
For example, when the number of pixels is “n” along a row direction and the OLED device is driven as a full white, the driving current may refer to “nIpixel.” Therefore, a drop of the driving current may occur due to a line resistance of the power line VDD. Further, when a line resistance in each pixel refers to “Rpixel” and a driving current in each pixel refers to “Ipixel,” the drop of the driving current in an Nth row of the row direction may refer to [n(n+1)/2]Rpixel*Ipixel. Therefore, since a gate voltage of each of the driving TFTs may be different from each other with respect to a same data voltage, a drop in the OLED current occurs. This drop of the driving current is increased with larger-sized OLEDs. Consequently, image quality degradation may be a problem.
In other words, the electric charge capacity charged in the storage capacitor Cst depends the gate voltage, so uniformity of the brightness may be depressed by changing the driving current applied to the OLED device. Accordingly, the drop capacity of the driving current is increased along the row direction, thereby reducing the brightness.
Moreover, problems such as brightness deviation due to the drop of the driving current may increase with larger and higher resolution OLED devices.